This invention relates to optical amplifiers and, more particularly, to such amplifiers in which the gain differs depending on whether the electromagnetic mode (i.e., polarization) of the radiation being amplified is transverse electric (TE) or transverse magnetic (TM). Included in this class of amplifiers are semiconductor optical amplifiers in which the radiation is guided in a waveguide having an asymmetric cross-section.
A major obstacle for the utilization of semiconductor optical amplifiers in lightwave communication systems is the polarization dependence of the optical gain. Typically, the difference in gain between the TE and TM modes in several dB. In the laboratory, the polarization can be easily controlled by twisting bulky fiber loops in the light path so that only one mode (TE) enters the amplifier. In the field, however, lightwave systems must operate unattended for long periods of time and twisted fiber loops are not principal; either active polarization controllers or polarization independent amplifiers are required. Because of the added complexity and cost associated with active polarization controllers, a polarization independent amplifier is the preferred approach.
One proposal for achieving polarization independence is to use two semiconductor amplifiers in parallel, one for each polarization. As described by G. Grobkopf et al, Electron Lett., Vol. 23, p. 1387 (1987), the input radiation, which is composed of both TE and TM modes, is split into two paths, separately amplified, and then recombined. However, because of the added complexity of having two optical amplifiers and two couplers, it would be advantageous to have a scheme that requires only one amplifier and one coupler.